Hydrosilylation catalyzed by

Scheme 10. Addition reaction by hydrosilylation catalyzed by a platinum complex.

Scheme 21 Hydrosilylation catalyzed by an Fe complex with phosphorus ligands...

Scheme 22 Asymmetric hydrosilylation catalyzed by the Fe complex with the pincer-type ligand...

A review8 with more than 186 references discusses the synthesis of Rh and Pd complexes with optically active P,N-bidentate ligands and their applications in homogeneous asymmetric catalysis. The effect of the nature of the P,N-bidentate compounds on the structure of the metal complexes and on enantioselectivity in catalysis was examined. Allylic substitution, cross-coup-ling, hydroboration and hydrosilylation catalyzed by Rh or Pd complexes with optically active P,N-bidentate ligands are considered. 

Recently, a proposal has been put forth that a /raor-addition process may be possible through dinuclear ruthenium intermediates.34 As shown in Scheme 5, reaction of tetraruthenium aggregate A with phenylacetylene results in the fully characterized bridging dinuclear alkenyl complex B. The authors propose a direct /ra .r-dclivcry of hydride through a dinuclear intermediate may be active in the hydrosilylation catalyzed by A, though compound B itself is unreactive to Et3SiH. 

Mechanism of Hydrosilylation Catalyzed by Surface versus Soluble Rhodium Siloxide Complexes... 

Diyne cyclization/hydrosilylation catalyzed by 4 was proposed to occur via a mechanism analogous to that proposed for nickel-catalyzed diyne cyclization/hydrosilylation (Scheme 4). It was worth noting that experimental evidence pointed to a silane-promoted reductive elimination pathway. In particular, reaction of dimethyl dipropargylmalonate with HSiMc2Et (3 equiv.) catalyzed by 4 led to predominant formation of the disilylated uncyclized compound 5 in 51% yield, whereas slow addition of HSiMe2Et to a mixture of the diyne and 4 led to predominant formation of silylated 1,2-dialkylidene cyclopentane 6 (Scheme 5). This and related observations were consistent with a mechanism involving silane-promoted G-H reductive elimination from alkenylrhodium hydride species Id to form silylated uncyclized products in competition with intramolecular carbometallation of Id to form cyclization/hydrosilylation products (Scheme 4). Silane-promoted reductive elimination could occur either via an oxidative addition/reductive elimination sequence involving an Rh(v) intermediate, or via a cr-bond metathesis pathway. 

Cyclization/hydrosilylation catalyzed by Cp 2YCH(SiMe3)2 failed in the case of dienes that possessed substitution on the alkenyl carbon atoms, presumably due to the excessive steric crowding about the yttrium center. In contrast. 

Palladium-catalyzed asymmetric cyclization/hydrosilylation tolerated a number of functional groups including benzyl and pivaloyl ethers as well as benzyl and methyl esters (Table 8, entries 1-4). Furthermore, the protocol tolerated substitution at one of the two /ra/zi -terminal alkenyl positions and at one of the two allylic positions of the 1,6-diene (Table 8). As was the case with diene cyclization/hydrosilylation catalyzed by achiral palladium... 

Organic-silica hybrid materials have been used for multi-photon microfabrication. These include the acrylate-functionalized oligosiloxanes known as ORMOCERs, which have been polymerized by radical processes using conventional IP radical iniatitors, such as c.2 [221,234]. Commercial poly(dimethylsiloxane)-based resists containing vinyl and Si-H functionalities use two different 2PA-induced processes hydrosilylation catalyzed by the photodecomposition products of Cp PtMes (Cp = ti -methylcyclopentadienyl) and radical initiation by c.4 (Fig. 13) [235]. The former process was complicated by thermally-induced polymerization. 

Migration of organosilicon groups from metal to ligand is believed to be a key step in the mechanism of olefin hydrosilylation catalyzed by nickel-phosphine complexes (59), as shown in Scheme 1. (Substituents are omitted for clarity.)... 

TABLE III Hydrosilylations Catalyzed by Supported Metal Catalysts ... 

Detailed studies concerning the active species in hydrosilylation catalyzed by chloroplatinic acid have pointed to the involvement of several oxidation states. It... 

The mechanism of the intramolecular hydrosilylation catalyzed by Rh and Pt complexes was investigated by using deuterated silanes, which indicated the operation of both the traditional Chalk-Harrod hydrometallation and silylmetallation pathways accompanied by rapid P-hydride elimination [65]. This intramolecular reaction was applied to the syntheses of natural products [66],... 

TABLE 6. Asymmetric reduction of prochiral imines via hydrosilylation catalyzed by (S,S)-(EBTHI)Ti complex" in THF... 

Hydrosilylation of carbonyl compounds. The definitive report on reduction of carbonyl compounds by hydrosilylation catalyzed by Wilkinson"scatalyst is available.1 Hydrosilylation can be used to effect regioselective 1,2- or 1,4-reduction of a,/ -enals or -enones by proper choice of the hydrosilane. In general, monohydrosilanes favor 1,4-adducts, whereas dihydrosilanes favor 1,2-adducts. The regioselectivity is also influenced by the substituents on silicon and on the substrates. The presence of a phenyl group on the enone system can effect dramatic changes in the selectivity. Examples ... 

Intramolecular hydrosilylation. The hydrodimethylsilyl ether (2) of a homo-propargyl alcohol (1) undergoes intramolecular hydrosilylation catalyzed by H2PtCl6 to give a cyclic (E)-vinylsilane (3). This can be converted to a 0-hydroxy ketone (4) by hydrogen peroxide oxidation or into (Z)-3-bromo-3-decene-l-ol (5) by brom-ination/bromodesilylation with inversion of geometry. 

However, it is now known that silyl migration is a very facile process, and in some cases both j8-silyl and j8-H migrations were found to be operative even at low (<-40°C) temperatures.33 The data for hydrosilylations catalyzed by CpRh(H)(SiR3)(C2H4) are in agreement with facile silyl migration and a Rhv intermediate 34... 

A related mechanism has been established for hydrosilylations catalyzed by cationic palladium complexes and is shown in Fig. 22-4. The mechanisms for both... 

Although hydrosilanes reduce ketones, in trifluoroacetic acid, to the corresponding methylene compounds or dimeric ethers via ionic hydrogenation, the reduction of a-amino and a-oxy ketones and p-keto acid derivatives with hydrosilanes, particularly PhMe2SiH, under these conditions proceeded with high anti selectivity to the alcohols. No racemization was observed at the carbon a to the carbonyl group. Intramolecular hydrosilylation catalyzed by Lewis acids provided a highly stereoselective route to anti-1,3-diols from p-hydroxy ketones (Section 1.1.3. ). ... 

The regioselectivity of the reaction is opposite to that observed in isoprene hydrosilylation catalyzed by other transition metal complexes. The product ratio depends on the structure of hydrosilane. 

The main side reaction of hydrosilylation catalyzed by transition metal complexes is dehydrogenative silylation ... 

The hydrosilylation of isoprene using RhCl(PPh3)3 as catalyst gives 3-methyl-2-butenylsilane as major product in addition to a lesser amount of (Z)-2-methyl-2-butenylsilane (equation 48)91. It is of interest to note that the regioselectivity of the reaction is opposite to that observed in isoprene hydrosilylation catalyzed by other transition metal complexes. The product ratio depends on the structure of hydrosilane. 

In the asymmetric hydrosilylation catalyzed by rhodium complexes, it has been shown that both configuration and optical yield of the resulting alcohols depend upon the structure of the hydrosilanes employed. Typical results are shown in Table 4. Copper(I)... 

Not surprising, the same protocol for H2/D2 scrambling can be employed in the hydrosilylation of carbonyl compounds [105]. Complexes 35 were found to be extremely active catalysts for the hydrosilylation of carbonyl compounds. As depicted in Scheme 48, the hydrosilylation catalyzed by 0.005-0.05 mol% of 38 was clean and efficient. Quantitative conversions can be achieved under mild conditions affording TONs up to 9,000 and TOFs up to 1.2 x 10 h Remarkably,... 

Preparation of chiral benzylic alcohols has been achieved by asymmetric dihy-drosilylation of terminal arylalkynes, followed by oxidation [24]. The chiral diol 85 with 95 % ee was obtained by direct Pd-catalyzed asymmetric dihydrosilylation of phenylacetylene (82) with HSiCl3 using MOP [(/ )-VI-18], followed by oxidation of 1,2-disilylated product 84, but the yield of 84 was 33 % (the main product was l-trichlorosilyl-2,4-diphenyl-l,3-butadiene). Better results were obtained by hydrosilylation using two catalysts. The first Pt-catalyzed monosilylation to give 83 was followed by the second hydrosilylation catalyzed by the chiral Pd catalyst. The diol 85 with 95 % ee was obtained in 87 % overall yield by this method. 

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